I believe Sharepoint will Surface Smartforms in an Iframe and as such overwriting the CSS within the iframe may not be possible:
http://stackoverflow.com/questions/6494721/css-override-body-style-for-content-in-iframe
As such the CSS changes will likely need to be for the Smartforms Sites. But if you modify the css for Plantinum, then any form that uses platinum will be affected. So I believe the only only would be to perhaps create a copy of Plantinum, but customize it for that site, i.e. PlatinumHR, PlatinumSales
That way, it is a customized Platinum, that can be applied per Forms:
http://help.k2.com/onlinehelp/K2smartforms/DevRef/4.6.11/default.htm#Custom_Themes.htm